The present invention relates to high intensity discharge lamps, and more particularly high intensity discharge lamps having multiple discharge devices.
High intensity discharge (HID) lamps generally include a discharge device having a translucent or transparent vessel that contains an ionizable material. Additionally, the discharge device may include a pair of discharge electrodes within the discharge vessel. In operation, an electrical discharge is developed within the ionizable material and emits light. For example, high pressure sodium discharge lamps have a mercury-sodium amalgam within the discharge vessel, and an inert starting gas. The starting gas is ionized to vaporize some of the amalgam, the vaporized mercury and sodium are ionized, and an intense light-emitting electrical discharge is formed.
The difficulty in restarting an HID lamp after the discharge has been interrupted is well-known. During operation of a typical HID lamp the gas pressure within the discharge vessel attains several atmospheres. When the discharge is interrupted the high internal pressure makes it difficult to reinitiate the discharge, i.e. restart the lamp. Consequently, the discharge vessel must cool until its internal pressure has dropped sufficiently to allow the discharge to be reestablished. The time required for the discharge tube to cool sufficiently to reestablish the discharge is called the hot restart delay.
Because the time required for a hot discharge vessel to cool and the internal pressure to drop sufficiently to allow the lamp to start may be several minutes, or even longer, an element of unreliability is present in lighting systems that rely solely on HID lamps. A momentary power interruption can result in an interruption in lighting service that lasts much longer than the power interruption. In applications where an interruption in lighting service cannot be tolerated, it is necessary to provide auxiliary lighting to furnish light until the HID lamps cool sufficiently to restart.
The problem of providing light during the hot restart delay has been addressed in different ways. One approach is to provide auxiliary incandescent lamps; however, such auxiliary lamps also require control circuitry for operation during periods of hot restart delay. This auxiliary equipment creates additional expense and creates a more elaborate system that that required for just operating the HID lamps.
Another approach to the hot restart problem is the use of more than one discharge device within the same lamp. U.S. Pat. No. 4,287,454 to Feuersanger et al discloses a high pressure discharge lamp in which a pair of lamp discharge devices are contained within the same lamp and connected in parallel. When a starting voltage is applied to the lamp one of the discharge devices operates first, and effectively shunts the second discharge device with a low impedance thereby preventing it from operating. Only the operative discharge device heats sufficiently to elevate its internal pressure and increase its starting voltage. Consequently, if the operating voltage applied to the lamp is interrupted, a reapplied voltage will start the previously inoperative discharge device which will operate without experiencing a hot restart delay.
The hot restart problem has also been addressed by the use of an auxiliary discharge gap exterior to an HID lamp discharge device, but within the lamp outer envelope. A lamp having this structure is disclosed in U.S. Pat. No. 4,377,772 issued to Tsuchihashi et al. This lamp is started in the usual fashion when an operating voltage is applied. After the normal temperature rise in the discharge device, the discharge device will exhibit the usual hot restart delay, if the lamp voltage is interrupted. In this case, reapplication of the lamp voltage during the period of hot restart delay will cause a discharge across the auxiliary discharge gap within the lamp envelope. The discharge across the auxiliary gap will continue until the discharge device cools sufficiently to restart, at which time the voltage drop across the discharge device will decrease and the voltage across the auxiliary discharge gap will be insufficient to continue the auxiliary discharge.
Optical shading caused by one discharge tube blocking the light from another does not occur in the prior art lamp having a single discharge tube and an auxiliary discharge gap within the lamp envelope. For lamps having more than one discharge tube, optical shading is avoided by positioning the discharge tubes aligned axially. This arrangement of discharge tubes is not practicable for high wattage lamps. For example, a 400 watt high pressure sodium HID lamp typically has a discharge vessel about 4.5 inches long and a lamp outer envelope almost eight inches long. In such a lamp having multiple discharge tubes it would not be practicable to axially align the discharge tubes. Optical shading of one discharge device by another is unavoidable when multiple discharge devices are arranged side by side.
It would be an advantage to luminaire design if HID lamps having multiple discharge devices could be made so that the same discharge device always started first, when the lamp is started from a cool condition. In this case the luminaire could be designed to minimize the effect of optical shading by the inoperative discharge device and maximize the use of the light emitted by the preferentially starting discharge device.